Abundance, diversity and foraging contribution of bumble bees to blueberry production in western Oregon

In Oregon, blueberry production has increased steadily in the last decade. For pollination, blueberry producers typically use commercial honey bees but their efficiency is limited since they do not buzz pollinate and do not forage under the cold and windy conditions that prevail during blueberry bloom in Oregon. Bumble bees are considered to be efficient pollinators of blueberries but there is limited information about their presence in blueberry fields in Oregon. Our objectives were to conduct a census of bumble bees during blueberry bloom, determine which species forage on blueberry flowers, and estimate their blossom visitation rate. Blue vane traps used for the census study captured 270 bumble bees belonging to 7 species. The same 7 species were also recorded during 127 sets of 2 min visual observations made while walking along rows of blueberry bushes. Bombus vosnesenskii was the dominant species comprising 52.5% of the 225 bumble bees observed foraging on blueberry blossoms. We recorded an average of 0.11 bumble bees per bush per 2 min of observation or 150 per ha per minute. Over 80% of bumble bees observed were queens, which is beneficial for blueberry pollination as queens are more efficient as pollinators than workers. Observations on flower visitations indicated that queens spent an average of 4 seconds per flower. Using bumble bee abundance, and floral visitation rates, we conservatively estimated that over 11.5 million flowers were visited during blueberry bloom. The abundance of native bumble bees and the high proportion of queens foraging on blueberries could account for the high yield (> 10 ton /acre) recorded by the producer from the study site though other native bees and honey bees could also have played a role.